Chapeter 4 Classes and Interfaces

Item 19: Use interfaces only to define types

When a class implements an interface, the interface serves as a type that can be used to refer to instances of the class.

1.constant interface: fails this test

[Conclustion] The constant interface pattern is a poor use of interfaces.
[What is it?] It contains no methods and consists solely of static final fields.
[Consequence] causes this implementation detail to leak into the class’s exported API
[Java platform libraries] such as java.io.ObjectStreamConstants

[Best Solution] If the constants are strongly tied to an existing class or interface, you should add them to the class or interface. Example: export MIN_VALUE and MAX_VALUE constants.

(1) export constants with enum type (item 30)

(2) export constants with a noninstantiable utility class (item 4)

          ===========================Constant Interface===========================
          // Constant interface antipattern - do not use!
          public interface PhysicalConstants {
              // Avogadro's number (1/mol)
              static final double AVOGADROS_NUMBER = 6.02214199e23;
						
              // Boltzmann constant (J/K)
              static final double BOLTZMANN_CONSTANT = 1.3806503e-23;
						
              // Mass of the electron (kg)
              static final double ELECTRON_MASS = 9.10938188e-31;
          }
          ==========================Constant utility class========================
          // Constant utility class
          package com.effectivejava.science;
          public class PhysicalConstants {
              private PhysicalConstants() { }
              // Prevents instantiation
              public static final double AVOGADROS_NUMBER = 6.02214199e23;
              public static final double BOLTZMANN_CONSTANT = 1.3806503e-23;
              public static final double ELECTRON_MASS = 9.10938188e-31;
          }
          -----------------------using constant utility class----------------------
          // a client should qualify constant names with a class name like 
          // PhysicalConstants.AVOGADROS_NUMBER .

          // [Alternative] as of release 1.5, using static import
          // Use of static import to avoid qualifying constants
          import static com.effectivejava.science.PhysicalConstants.*;

          public class Test {
              double atoms(double mols) {
                  return AVOGADROS_NUMBER * mols;
              }
              ...
              // Many more uses of PhysicalConstants justify static import
          }

Conclusion
- interfaces should be used only to define types. They should not be used to export constants.

Item 19: Use interfaces only to define types

1.constant interface: fails this test

Conclusion